The invention relates to a proximity switch with a coil arrangement having at least one coil and an evaluation circuit arranged downstream of the said coil arrangement for obtaining a switching signal when a trip feature of the coil arrangement approaches the desired response distance.
Proximity switches of this type are known in the prior art.
Inductive proximity switches in particular can be divided into two groups. There are what are known as non-flush-mountable proximity switches, which have the greatest possible operating distance with respect to the device diameter, and consequently coil diameter. These proximity switches must have around their active surface a certain free zone for metallic parts, in order not to change their operating distance significantly during mounting. If such proximity switches are fitted in the direct vicinity of metal masses, they lose their switching capability. The advantage of non-flush-mountable proximity switches of this type is their great operating distance.
On the other hand, there are flush-mountable proximity switches. In the case of these proximity switches, an internal shielding of the coil system provides an insensitivity to metal masses arranged alongside the proximity switch. This safeguard against unwanted influences is obtained at the expense of the disadvantage that their operating distance is considerably less than that of the non-flush-mountable proximity switches.
A proximity switch in which an alternating magnetic field is generated by a coil, which coil is located between two differential coils, is shown by DE 40 31 252. A differential voltage is induced in the two receiving coils and returned to the input of an oscillator amplifier. This voltage becomes zero at precisely that moment at which the object to be sensed is at the desired operating distance. The coil combination must be adapted specifically to the operating point with regard to its number of turns and its spatial position. In the case of a proximity switch of this type, the oscillator changes its oscillating state abruptly, which is detected by an evaluation circuit.
Also known in the prior art are inductive proximity switches in which the coil is part of an LC oscillator. The alternating electromagnetic field produced by the coil generates eddy currents in an approaching ferromagnetic object to be sensed. These currents draw energy from the oscillator. This leads to a reduction in the oscillation amplitude of the oscillator. This is detected by the evaluation circuit.
All the proximity switches described above share the problem that a shielding of the coil arrangement is needed if they are disposed in the vicinity of metal masses (flush mounting), which is accompanied by the disadvantage that the operating distance is reduced.